Radio frequency (RF) devices, e.g., RF transistors and RF power amplifiers (“PAs”) containing RF power transistors, are used in a wide variety of communications and other electronic applications, such as cellular handsets and base radio repeaters. RF power amplifiers are typically made up of one or more cascaded amplifier stages, each of which increases the level of the signal applied to the input of that stage by an amount known as the gain of the stage. It should be noted that the terms RF device, PA device and power transistor device are used herein interchangeably to describe a device that includes one or more power transistors operating as a power amplifier in an application.
In recent years, manufactures of PA apparatus that includes a PA device comprising one or more power transistors have generally focused efforts on increasing the maximum operating frequency of those devices to accommodate higher frequency markets such as, for instance, 2 GHz markets. Companies that develop applications that span a range of operating frequencies may, for reasons such as leveraging volume and cost considerations, desire to use a single PA design, transistor or power-integrated circuit (IC) ‘device’ for all or a significant portion of their applications. However, designers are confronted with a new challenge when using readily available power amplifier devices in “lower frequency designs” than what the power amplifier or device is otherwise capable of or intended for. A power transistor device within the power amplifier can self-destruct as the amplifier output power is increased. This self-destruction phenomenon is also referred to as a “lack-of-ruggedness” and can occur at power levels of varying degrees below the power level at which the device is rated.
FIGS. 1 and 2 will be used to further describe this self-destruction phenomenon. FIG. 1 illustrates exemplary power amplifier apparatus 100 as described above that may be used in applications having a maximum operating frequency of 1 GHz for instance. Apparatus 100 comprises at least one power transistor 110 that functions as a power amplifier and a matching network 120 connected to the output of transistor 110. In this illustrated embodiment, power transistor 110 is a power field effect transistor (FET) such as a lateral diffused metal-oxide-semiconductor (LDMOS) transistor having a drain at a node 112, a gate at a node 114 and a source at a node 116. The matching network 120 is connected to nodes 112 and 116.
In operation, the gate of transistor 110 is coupled to an RF input waveform at a given fundamental or main frequency, which typically comprises an RF carrier modulated with information to be communicated over the air or a cable. Transistor 110 then generates an amplified RF signal at its output, which creates a voltage response between the drain and source (Vds). Vds is a function of a drain current Id generated in the transistor and an impedance, which is based on matching network 120 as “seen” by Id. Generally, Id comprises a spectral component at the fundamental frequency (also referred to herein as the fundamental component) and spectral component(s) at one or more harmonic frequencies (also referred to herein as harmonic components), where a harmonic frequency is defined as an integral (whole-number) multiple of the fundamental frequency.
In a typical application, transistor 110 is operated near its maximum operating frequency. Therefore, it is usually sufficient to design matching network 120 to correspond to or “dominate” only the fundamental component of Id to cause the transistor to operate within whatever design specifications are required for the application. Dominate herein means to specifically or overtly load a given spectral component of the output current of a PA device with a desired impedance to create a corresponding desired spectral component of the voltage response at the output of the PA device. Those skilled in the art will realize that in a physical circuit impedance is affected by many factors. In addition, the impedance is the inverse of a complex admittance value, wherein the real part of the admittance value is directly related to the slope of the load line.
FIG. 2 illustrates an exemplary Vds output waveform 220 generated by transistor 110 over time where transistor 110 is being operated at a fundamental frequency that is substantially less than the maximum operating frequency of the transistor. In this illustration, transistor 110 has a Vds specification (or maximum breakdown voltage rating) of about 65 volts and is being operating at a fundamental frequency in the VHF frequency range, in this case about 146 MHz. However, as FIG. 2 shows, operating the transistor in this manner generates a Vds output waveform 220 comprising high instantaneous voltage peaks (e.g., 222 and 224) that exceed the maximum Vds voltage specification of the transistor. These voltage peaks are caused by the additive effects of harmonic component(s) of Id, in turn generating harmonic Vds components, and are what causes transistor 110 to self-destruct when it is being operated at certain frequencies that are less than the maximum operating frequency of the transistor. In other words, the power transistor has available gain at a number of harmonics, which may create the undesirable time-domain RF voltage waveform. Currently, there exists no suitable method for the application designer to address this lack-of-ruggedness problem.
Embodiments of the present invention as described below in detail provide for techniques that address the need for eliminating elevated peaks in the Vds output voltage of a power transistor that exceed the maximum break down voltage of the transistor to enable it to be operated for a larger range of frequencies that are substantially lower than a given predetermined maximum operating frequency of the transistor.